Vegetable oils usually used as food oils include soybean oil, rapeseed oil, coconut oil, sesame oil and the like. In producing such plant oils, depending on the amount of oil contained therein, a raw material is pressed or the feed material is extracted with an organic solvent (such as hexane) to obtain miscella, and, then, the organic solvent in the miscella is distilled away to yield a crude glyceride oil composition. Such a crude glyceride oil composition generally contains phospholipid, saccharide organic fatty acid, amino acid, protein, dye material and the like as unavoidable impurities. Of these impurities, phospholipid must absolutely be removed for the reasons described below. The crude glyceride oil composition usually contains 0.5 to 3 wt% of gum material composed mainly of phospholipids, such as lecithin, which gum material is decomposed on heating the oil to result in oil coloration, the generation of unpleasant odors and a deterioration in the taste of the oil. It is necessary, therefore, to remove such gum material from the crude glyceride oil composition as much as is possible.
In accordance with conventional purification methods, water is added to the crude glyceride oil composition to hydrate the gum material and after the same is coalesced, the gum material is removed by centrifugal separation. The thus-purified glyceride oil composition will still contain about 0.2 to 0.4 wt% of gum material. Usually, therefore, gum removal is repeated using chemicals, followed by treatment to remove color, acids and odors to obtain a glyceride oil product having a gum content of 100 ppm or less. Thus, conventional purification methods suffer from the disadvantages in that they involved repeated complicated gum removal operations and a considerable amount of oil is inevitably lost during such operations.
In order to overcome the disadvantages of such conventional purification methods, a process for the purification of crude glyceride oil compositions was proposed in U.S. Pat. No. 4,062,882. In accordance with this process, a crude glyceride oil composition is diluted with an organic solvent (such as hexane) to prepare miscella, the thus-prepared miscella is brought into contact with a synthetic polymeric semipermeable membrane under pressure to separate it into a membrane permeable solution composed of hexane and glyceride oil and a concentrated membrane impermeable material, whereafter the hexane is distilled from the membrane permeable solution to obtain a purified oil. As the concentrated membrane impermeable material contains high concentrations of phospholipids, such as lecithin, they may be recovered therefrom.
Various problems, however, arise in the production of purified glyceride oil on a commercial scale by membrane treatment of crude glyceride oil compositions per the above proposed process.
In order to obtain a purified glyceride oil as a membrane permeable solution at a high recovery rate via membrane treatment of miscella, it is usually necessary to concentrate the miscella to about 50 to 60 times original concentration. The viscosity of the miscella increases as it is concentrated, and depending on the form in which the semipermeable membrane is used, it is impossible to concentrate the miscella to such high levels. For example, internal pressure type capillary semipermeable membranes having a small pipe cross-sectional area, e.g., an inner diameter of about 0.1 to 2 mm, are advantageous for miniaturization of equipment since they have a high membrane area per unit volume. With such semipermeable membranes, however, it is possible to concentrate only to about 10 times the original concentration, and the miscella still has a relatively low viscosity.
The amount of membrane permeable solution per unit membrane area is highly influenced by the pressure applied when the miscella is brought into contact with the membrane. In the case of internal pressure type capillary semipermeable membranes, however, it is generally impossible to reinforce them with an unwoven fabric tube, etc., and furthermore, the membrane walls are relatively thin, in distinction to hollow filament membranes. Since the pressure of the supplied miscella must be supported by such thin membranes without reinforcement, the maximum pressure that can be applied is only about 5 kg/cm.sup.2. Therefore, when the viscosity of the miscella is increased, it is not possible to increase the amount of the membrane permeable solution by raising the pressure at which the miscella is supplied. Thus, purification of crude glyceride oil compositions by a one stage membrane treatment using capillary semipermeable membranes requires a long period of time and can be performed on a commercial scale only with difficulty.
On the other hand, the use of internal pressure type tubular semipermeable membranes having an inner diameter of, e.g., about 2 to 20 mm, permits one to concentrate the miscella about 50 to 100 times original concentration since they have a large tube cross-sectional area. Furthermore, since such tubular membranes are generally reinforced with a paper tube, an unwoven fabric tube, a porous polymer tube, a porous metallic tube or the like, it is possible to apply a pressure as high as about 80 kg/cm.sup.2 which permits one to increase the amount of membrane permeable solution even with miscella having a high viscosity. However, the purification of crude glyceride oil compositions by a one stage membrane treatment using tubular semipermeable membranes requires large size equipment and the number of necessary membrane modules is markedly increased, which is undesirable from an economic viewpoint. Thus, the use of semipermeable membranes in a form which enables one to treat miscella having high viscosity significantly increases equipment size, is accompanied by an increase in purification costs and thus one loses many of the advantages of membrane treatment.
As a result of research regarding the above-described problems encountered in the purification of crude glyceride oil compositions with internal pressure type membrane modules, it has been found that by firstly treating the miscella of a crude glyceride oil composition with a capillary semipermeable membrane and then treating the thus-concentrated miscella with a tubular semipermeable membrane, the miscella can be highly concentrated and a purified glyceride oil can be obtained at a high recovery rate while minimizing the number of membrane modules necessary.